Colorimetric determination of iron

ABSTRACT

3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-1,2,4-triazine and certain salts thereof, e.g. sodium, are provided together with a method for forming such compounds. These compounds are useful as reagents in the spectrophotometric determination of iron in water or other solutions.

This is a division of application Ser. No. 49,579, filed June 24, 1970.

This relates to novel chemical compounds and, more particularly, tocompounds which may be advantageously employed in chemical analyses foriron.

There are a wide variety of applications wherein it is desirable todetermine the presence of iron in water or other solutions. For example,it is necessary to analyze for the iron content in the boiler water in asteam electric power-generating plant because the iron oxide content ofthe water is an index of the rate of corrosion taking place in theboiler.

In carrying out the conventional methods for analysis of iron, the firststep involves solubilizing the solid iron compounds in the test sample.This is typically achieved by adding an acid such as hydrochloric acidto the sample and then allowing sufficient time for the solid ironcompounds to dissolve. Because dissolution proceeds slowly at roomtemperatures, the sample is generally heated to boiling for the timenecessary to achieve solubilizing. A reagent such as hydroxylammoniumchloride is then added to reduce the iron to the ferrous form. Thislatter reagent is sometimes combined with the hydrochloric acid additionto minimize the number of steps involved.

A reagent is then added to the test sample solution (suitably bufferedto a pH value determined by the reagent being used as is well known)which will form a colored complex with the iron at the pH levelinvolved. The iron content is then determined colorimetrically by wellknown means. Commercial analyzers capable of continuously carrying outthe hereinbefore described method are available.

Compounds termed "ferroin" reagents have been widely used as thecolorimetric reagent for the iron analysis hereinbefore described. Theseorganic molecules, containing the atomic configuration. ##STR1## reactas bidentate ligands with certain metal ions including ferrous ions togive colored complexes. This effect was first noticed with the ferrousion; and, since the ferrous complexes are generally of a more intensecolor than with other metals, the atomic configuration was given thetrivial name of the "ferroin" group.

While hundreds of compounds containing the ferroin group have beensynthesized and the majority of these demonstrate the ability to formcomplexes with the ferrous ion, most of these are either only weaklycolored, or are unstable under normal physical conditions or are formedonly over a very narrow pH range.

A few of these compounds, however, form stable, highly colored specieswith the ferrous ion and are therefore suitable for use as colorimetricreagents for the quantitative determination of iron. Examples ofcompounds that have found acceptance are 1,10-phenanthroline;4,7-diphenyl-1, 10-phenanthroline; 2,2'-bipyridyl;2,6-bis(2-pyridyl)pyridine; 2,4,6-tris (2-pyridyl)-1,3,5-triazine; andphenyl 2-pyridyl ketoxime.

Most of these reagents are the products of difficult and tedious organicsyntheses together with the fact that the raw materials are relativelyexpensive. Accordingly, while the cost of a manual classical laboratoryanalysis is fairly small due to the small quantity of reagent involved,the cost of employing such expensive ingredients in operating anautomatic continuous analyzer would be prohibitive.

Recently, Case reported the preparation of 24 new compounds (30 J. Org.Chem., 931, 1965) which may have some application as chromogens foriron, cobalt and copper. These were later tested by Schilt (13 Tatanta,895-902, 1966).

Among these compounds was a substituted triazine, i.e.3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine. While this material is a goodreagent for the determination of iron, it, like other high molecularweight compounds containing the ferroin group, suffers the disadvantageof being quite insoluble in water as is the complex which it forms withthe ferrous ion.

While this insolubility does not present any problems in carrying out alaboratory analysis for the determination of iron, the insolubilityeliminates, from a practical standpoint, the compound as a reagent foruse as a reagent in a completely automated analysis.

It is accordingly an object of the present invention to provide asensitive spectrophotometric reagent for iron which may beadvantageously employed in completely automated analysis techniques. Arelated and more specific object provides novel ferroin compounds whichare water soluble.

A further object lies in the provision of an economical method forforming a spectrophotometric reagent for iron.

A still further object provides a spectrophotometric reagent of theabove-identified type which is stable, both in air and in an aqueoussolution.

Another object is to provide a reagent of the above-identified typewhich may carry out the determination with only a minimum ofinterferences.

Other objects and advantages of the present invention will becomeapparent as the following description proceeds.

While the invention is susceptible of various modifications andalternative forms, certain specific embodiments thereof have been shownby way of examples which will be described in detail herein. It shouldbe understood, however, that it is not intended to limit the inventionto the particular forms disclosed, but, on the contrary, the intentionis to cover all modifications, equivalents and alternatives fallingwithin the spirit and scope of the invention.

Briefly, the present invention provides a spectrophotometric reagentcomprising 3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-1,2,4-triazine orcertain soluble salts thereof such as the disodium, the diammonium orthe dipotassium salt, the corresponding mono salts, or mixtures thereof.The principal product has the sulfuric acid groups at the 4-position andthese have the following structural formula: ##STR2## wherein x is amember selected from the group consisting of hydrogen, sodium, potassiumor ammonium. The sensitivity of the subject reagent, measured as themolar extinction coefficient at the wavelength of maximum absorbance(i.e. 562 nm.) is greater than the previously described compoundprepared by Case as well as other prior ferroin reagents. Thus, itssensitivity is 27,800 (moles⁻¹ cm⁻¹) as compared with values of 11,100for 1,10-phenanthroline, 22,143 for 4,7-diphenyl-1, 10-phenanthroline,and 22,600 for 2,4,6-tris(2-pyridyl) 1,3,5-triazine.

In addition, the ferrous complex is stable over a wide pH range, i.e.from about 4 to about 9; and, once the complex is formed in this range,the solution may be made 1 N with respect to perchloric, nitric, orhydrochloric acids without any noticeable fading taking place forseveral hours.

The reagent itself is stable in air and in aqueous solution and is notsusceptible to chemical destruction except under the most drasticconditions. Positive interferences are limited to cuprous and cobaltousions, both of which form slightly yellow complexes and do notsubstantially interfere until they are present in large excess over theferrous ion. Negative interferences occur only with cyanide andhydrosulfite in large concentration.

To prepare the subject reagent in accordance with the present invention,2-pyridyl hydrazidine, a first intermediate, is formed. This is achievedby providing a reaction mixture of hydrazine (preferably anhydrous) and2-cyanopyridine (both relatively inexpensive commercially availablecompounds) in a solvent such as isopropanol (again preferablyanhydrous). The reaction mixture is stirred at room temperature forabout 4 hours. The proportions are not critical; however, it ispreferred to employ an excess of hydrazine (up to about 100 percent ormore) so that the reaction will go to substantial completion. Thereaction mixture is then cooled to from about 0° to 10° C. with theintermediate, 2-pyridyl hydrazidine, crystallizing out as a whiteprecipitate.

The hydrazidine intermediate is then treated to remove any hydrazineimpurities. This may be typically achieved by washing with a solvent,suitably ice cold isopropanol. Removal of the impurity is believed toprevent discoloration of the subsequent intermediate which mightotherwise occur.

This intermediate is then dissolved in a solvent such as isopropanol anda stoichiometric amount of benzil is dissolved in a separate solvent,which may again be isopropanol. Using an excess of benzil is notpreferred since it could conceivably contaminate the product and causedecomposition during subsequent steps, such as sulfonation. Both ofthese reactant solutions may be heated to 40° to 60° C. to increase theamount of reactant which may be placed in the given amount of solventwhich is being employed. 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine isthen formed by adding the two reactant solutions to each other.

Typically, this is carried out by adding the hydrazidine to the benzilwith stirring. The reaction is slightly exothermic and within a fewminutes a white precipitate begins to form (a yellow precipitate may beformed if not all the hydrazine was removed or the temperature involvedin preparing the hydrazidine was too high). After about 30 to 60 minuteswith stirring, the formed precipitate is filtered and washed at roomtemperature with isopropanol to free the precipitate of any unreactedmaterials. This material is then dried at room temperature and passedthrough a suitable mesh screen to provide uniform sized particles (e.g.20 mesh).

In accordance with the present invention, the reagent compounds of thepresent invention are then formed by sulfonating the ground material.This is typically carried out by adding at least a stoichiometric amountof fuming sulfuric acid to the ground material at room temperature withstirring. The reaction is exothermic and will raise the reactiontemperature to about 120° C. and the partially sulfonated product willgo into solution quite rapidly.

At this point, and in accordance with another aspect of the presentinvention, additional heat is provided to bring the temperature of thereaction mixture up to about 200° C. (±5° C.) in as little time aspossible. If the temperature of the reaction mixture is not broughtwithin this range, complete sulfonation will not result. Additionally,if the temperature is not brought rapidly to this temperature, theprolonged contact at elevated temperatures with the fuming sulfuric acidwill result in degradation of the organic intermediate. The timinginvolved in raising the temperature to about 200° C. may be up to about30 minutes or so; however, it is preferred to bring the temperature upto this point within about 15 to 20 minutes or even less.

This reaction is not time dependent; and, after the reaction temperaturehas been advanced to 200° C. or thereabouts, the external heat isremoved and the reaction mixture is cooled to about 30° to 50° C. asquickly as possible. The existence of the disulfonic acid may beconfirmed by organic microanalysis. Infared evidence suggests that thesulfonic acid groups occupy the 4-position and this is the majority ofthe product. It should be appreciated that minor portions of the productmay have the sulfonic acid groups at other positions.

When it is desired to form a salt, the resulting soluble product maythen be added to a salt solution which is very nearly saturated. Forexample, when the solution is a sodium chloride brine, a light yellowprecipitate will form over a period of time, which is predominantly thedisodium salt. Minor impurities such as sodium chloride and sodiumsulfate may be also formed but these do not interfere with the functionof the disodium organic salt as a spectrophotometric reagent for iron.These impurities may, however, be easily removed by crystallizing fromwater.

Further processing typically involves drying the precipitate as a cakeand then grinding up. For use as a reagent, the ground-up product maythen be dissolved in a quantity of water to form an aqueous reagent.

The following example is merely exemplary as to how the presentinvention could be carried out. It is representative of this inventionand should not be considered in limitation thereof.

EXAMPLE

2.0 liters of isopropanol, 600 grams of anhydrous hydrazine and 678grams of 2-cyanopyridine are added to a 5-liter round-bottomed flask.The resulting reaction mixture is stirred for about 4 hours. Thereaction mixture is then cooled until its temperature is below about 15°C. The cooled reaction mixture is then filtered through a medium filterpaper and the white crystals of 2-pyridylhydrazidine are washedthoroughly with isopropanol at 0° C. to remove all possible hydrazine.

1370 grams of benzil are then dissolved in 15 liters of is opropanol,using a 22-liter flask with mantle and stirring. Heating to atemperature of from about 50° to 55° C. should be sufficient. All the2-pyridylhydrazidine is dissolved in a minimum amount of isopropanol(about 5-liters), with heating only if necessary.

The solution of the 2-pyridylhydrazidine is then added to the solutionof the benzil with stirring for about 1 hour. A precipitate should formwithin several minutes.

The reaction mixture is filtered through a medium filter paper. Theresulting cake of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine is rinsedwell with isopropanol and then air dried at room temperature.

The 3-(2-pyridyl) 5,6-diphenyl-1,2,4-triazine (about 2 kilograms) isplaced in a 5-liter round-bottomed flask with a wide neck. 3 liters offuming sulphuric acid is added to the flask. The solution is heated toabout 200° C. with an open flame as quickly as possible. The heating isthen ceased and a small fan directed against the flask which is stirredconstantly so that the temperature drops several degrees per minute.

When the temperature has fallen below about 50° C., 14 liters ofsaturated sodium chloride solution in a 22-liter flask is prepared.While stirring the sodium chloride solution, the contents of the 5-literflask are slowly added to the flask containing the sodium chloridesolution via a separatory or additional funnel. The addition should takeabout 2 hours.

The resulting suspension is then filtered through medium paper, the cakedried and rinsed with methanol. The final product represents the sodiumsalts of 3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-1,2,4-triazine, theprincipal portion being the disodium salt.

Thus, as has been seen, a spectrophotometric reagent for ferrous ironhas been provided by the present invention which is soluble in water,may be economically formed from readily available raw materials andwhich functions with a minimum of interferences. The reagent is amplysuited to either manual analysis or continuous automatic instrumentalanalysis for on-stream monitoring. The sensitivity of the reagentexceeds that of any of the commercially available iron reagents.

I claim as my invention:
 1. In a method for quantitatively determiningthe presence of iron in a sample solution which includes adding areagent to a sample solution containing iron in the ferrous state toform a colored complex and colorimetrically determining the ironcontent, the improvement comprising using3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-1,2,4 triazine as the reagent.Iadd.to provide greater sensitivity as compared to that obtained with3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine.Iaddend..
 2. The method ofclaim 1 wherein the sulfonic acid groups are located at the 4-position.